Merge pull request #11157 from airspeedswift/stringprotocolify

[stdlib] Switch Int/Float inits + join to be on StringProtocol

stdlib/public/core/String.swift

@@ -873,7 +873,7 @@ extension String {
   }
 }
 
-extension Sequence where Element == String {
+extension Sequence where Element: StringProtocol {
 
   /// Returns a new string by concatenating the elements of the sequence,
   /// adding the given separator between each element.
@@ -907,7 +907,7 @@ extension Sequence where Element == String {
       for chunk in self {
         // FIXME(performance): this code assumes UTF-16 in-memory representation.
         // It should be switched to low-level APIs.
-        r += separatorSize + chunk.utf16.count
+        r += separatorSize + chunk._ephemeralString.utf16.count
       }
       return r - separatorSize
     }
@@ -918,17 +918,17 @@ extension Sequence where Element == String {
 
     if separatorSize == 0 {
       for x in self {
-        result.append(x)
+        result.append(x._ephemeralString)
       }
       return result
     }
 
     var iter = makeIterator()
     if let first = iter.next() {
-      result.append(first)
+      result.append(first._ephemeralString)
       while let next = iter.next() {
         result.append(separator)
-        result.append(next)
+        result.append(next._ephemeralString)
       }
     }